1. Field of the Invention
The invention relates to a technical field of a crystal device for surface mounting which is appropriate for miniaturization, and in particular, to a ceramic case 1 having end face electrodes for forming solder fillets thereon.
2. Description of the Related Art
A crystal device for surface mounting such as a crystal unit, a crystal oscillator, or a crystal filter is known as a frequency control element. For example, a crystal oscillator for surface mounting (hereinafter, referred to as a surface-mounted oscillator) is built into various types of electronics as a source of frequency or as a time reference. In recent years, the miniaturization thereof has been further advanced, which has brought about their planar outlines which are 2.0×1.6 mm or less, for example.
FIGS. 3A to 3C are diagrams for explanation of one example of a related art surface-mounted oscillator. FIG. 3A is a cross-sectional view of the related art surface-mounted oscillator, FIG. 3B is a plan view of the relate art surface-mounted oscillator without a metal cover, and FIG. 3C is a plan view of the related art surface-mounted oscillator to which the metal cover is jointed.
The related art surface-mounted oscillator 200 is configured such that, a crystal element 2 and an IC chip 3 are housed in a ceramic case 1 in a rectangular form in plan view whose cross section is formed to be concave and which has an inner wall shoulder, and a metal cover 4 is jointed to the ceramic case 1 to hermetically encapsulate the crystal element 2 and the IC chip 3. The ceramic case 1 includes a bottom wall layer 1a, a frame wall intermediate layer 1b, and a frame wall upper layer 1c. Mounting terminals 5 are provided to the four corners of the outer bottom face in the ceramic case 1 (bottom wall layer 1a), that are extended as end face electrodes 5a for solder fillets on the corners of the bottom wall layer 1a. 
Generally, the ceramic case 1 is formed such that, as shown in an enlarged partial plan view of FIG. 4, in a state of a ceramic green sheet, a sheet-like bottom wall layer 11a, a sheet-like frame wall intermediate layer 11b, and a sheet-like frame wall upper layer 11c are laminated to form a sheet-like ceramic case 11, and thereafter, the sheet-like ceramic case 11 is divided into separate ceramic cases 1. Through holes 6 (6a, 6b and 6c) passing through the both principal surfaces are formed in the respective intersections on the parting lines shown by A-A and B-B of the sheet-like ceramic case 11.
In this case, the through holes 6 of the sheet-like ceramic case 11 are formed such that a radius of curvature of a through hole 6a in the sheet-like bottom wall layer 11a is greater than a radius of curvature of through holes 6b and 6c in the sheet-like frame wall intermediate layer 11b and the sheet-like frame wall upper layer 11c. Then, a base electrode of, for example, W (tungsten) or the like, in a circuit pattern (not shown) is formed on the sheet-like bottom wall layer 11a, the sheet-like frame wall intermediate layer 11b, and the sheet-like frame wall upper layer 11c by printing. At this time, the base electrode is also formed on the inner circumferential surfaces of the through holes 6a in the sheet-like bottom wall layer 11a. 
Thereafter, after the sheet-like bottom wall layer 11a, the sheet-like frame wall intermediate layer 11b, and the sheet-like frame wall upper layer 11c are laminated, the sheet-like ceramic case 11 is submerged into an electrolyte, and Ni (nickel) and Au (gold) layers are serially plated onto the base electrode. Thereby, plated layers are formed on the base electrode exposed on the surface of the sheet-like ceramic case 11.
Then, the end face electrodes 5a for forming solder fillets thereon, which are extended from the mounting terminals 5, are formed in the respective through holes 6a in the sheet-like bottom wall layer 11a. Thereafter, the sheet-like ceramic case 11 is divided to obtain ceramic cases 1 having arc-formed notched parts 6′ into which the through holes 6 are respectively divided into four on their outer circumference four corners.
Since the through holes 6a in the sheet-like bottom wall layer 11a are made for solder fillets, the through holes 6a are made greater than the through holes 6b and 6c in the sheet-like frame wall intermediate layer 11b and the sheet-like frame wall upper layer 11c, to make a surface area of the divided end face electrodes greater. Further, the through holes 6b and 6c in the sheet-like frame wall intermediate layer 11b and the sheet-like frame wall upper layer 11c make the electrolyte easy to flow therein, and prevent burrs from occurring, for example, when the sheet-like ceramic case 11 is divided.
The crystal element has leading electrodes 7 on the both principal surfaces, and extractor electrodes 8 are extended on the both sides of one end thereof. The extended both sides of the one end of the extractor electrodes 8 are fixed to the inner wall shoulder (frame wall intermediate layer 1b) in the ceramic case 1 by an electrically conductive adhesive 9. The IC chip 3 in which at least oscillator circuits are integrated is fixed to the inner bottom face of the ceramic case 1 by flip-chip bonding.
The metal cover 4 is seam-welded to a metal ring 10 provided on an end face of the opening (surface of the frame wall upper layer 1c) of the ceramic case 1. A metal film 13 is provided to the end face of the opening of the ceramic case 1, and the metal ring 10 is connected to the metal film 13. Normally, the metal film 13 is connected to earth or ground terminals in the mounting terminals 5 provided on the outer bottom face via through holes (electrode through holes) (not shown), to be so-called case grounding (see, for example, JP-A-2006-303919). Incidentally, reference numerals 12 in the drawing denote crystal holding terminals. Further, the notched parts 6c′ (6b′) in FIGS. 3A and 3B are respectively illustrated so as to be different in size for descriptive purposes.
However, in the related art surface-mounted oscillator, the through holes 6b and 6c in the sheet-like frame wall intermediate layer 11b and the sheet-like frame wall upper layer 11c are smaller than the through hole 6a in the sheet-like bottom wall layer 11a, and the both have steps therebetween, which has caused the following problem. That is, when the sheet-like ceramic case 11 is plated in an electrolyte, the steps formed by the through holes 6a and 6b in the sheet-like bottom wall layer 11a and the sheet-like frame wall intermediate layer 11b generate a liquid pool of the electrolyte in the step portions.
Accordingly, the plated layer formed on the inner circumferential surfaces of the sheet-like bottom wall layer 11a is made greater in thickness on its step portions to cause sags or the like. Thus, for example, this makes it difficult to divide the sheet-like ceramic case 11 without causing burrs or the like, which may cause a defective appearance or dust after soldering. The smaller the planar outline of the surface-mounted oscillator (ceramic case 1) is made, the more difficult it is for the electrolyte to infiltrate the through holes 6b and 6c in the sheet-like frame wall intermediate layer 11b and the sheet-like frame wall upper layer 11c, which compounds the problem.